The invention relates to a photo enhanced CVD apparatus.
Many chemical vapor deposition (CVD) processes are used, such as APCVD, LP CVD, plasma CVD, thermal CVD and so forth, for depositing a film on a substrate. While these processes have their own peculiar characteristics respectively, the temperature at which each process is carried out is commonly rather high. Such high temperature process is not suitable for formation of passivation film on an aluminum electrode arrangement.
Photo enhanced CVD process has attracted the interest of artisans because it can be carried out at a comparatively low temperature. This process is based on the energy of light, namely an optical reaction is carried out. For example, in the case of photo CVD process using silane and ammonia, mercury atoms are excited by irradiation of ultraviolet light of 2,537 .ANG. in wavelength. The process is carried out to deposit a silicon nitride film on a substrate in accordance with the following equation: EQU Hg+hv.fwdarw.Hg* ("*" is a symbol for excitation) EQU Hg*+SiH.sub.4 .fwdarw.SiH.sub.3 +H-+Hg ("-" is a symbol for radical) EQU Hg*+NH.sub.3 .fwdarw.NH.sub.2 -+H-+Hg EQU yNH.sub.2 -+xSiH.sub.3 .fwdarw.Si.sub.x N.sub.y +zH.sub.2
In the above equations, x, y and z are chosen appropriately.
FIG. 1 is a cross-section view showing a photo CVD apparatus which has been devised by the inventors in advance of the present invention. To facilitate the understanding of the background of the present invention, this apparatus will be briefly explained. In the figure, the apparatus comprises a reaction chamber 31, light source chambers 39 and ultraviolet light sources 41. Between the light source chambers 39, a cart 35 is mounted so as to be capable of moving in the direction perpendicular to the drawing sheet. The cart is provided with heaters 37 to heat substrates mounted on the external surfaces of the cart 35 facing to the light source chambers 39. The temperature of the substrates 33 is elevated to about 200.degree. C. which is suitable for forming a silicon nitride film. In the reaction chamber 31 is circulated a process gas at a pressure of several Torrs. The process gas is irradiated through quartz windows 47 with light radiated from the light source 41. A numeral 45 designates electrodes by virtue of which discharge takes place with the cart as the other electrode and undesired product deposited on the surface of the quartz windows 47 can be eliminated by sputtering. However, with this apparatus, the thickness of deposited film depends on the spatial relationship between the light sources and the position of the substrates. Namely, the product of the CVD process may be deposited with a greater thickness at the position irradiated with stronger light. Generally speaking, the tolerable fluctuation of the thickness of the film is about 10%. Furthermore, the quartz windows 47 have to be thick to bear the differential pressure between the inside of the reaction chamber 31 and the light source chamber 39 in which cooling gas is circulated. The differential pressure may cause leakage of the cooling gas from the light source chamber 39 into the reaction chamber 31. As an alternative, a particular cooling system may be provided for the light source chamber so the pressure in the light source chamber, and therefore the differential pressure, can be decreased. Also, when discharge between the cart 35 and the reaction chamber 31 is desired to remove unnecessary film deposited on the light window by sputtering, the discharge tends to deviate from the window. Because of this, the particular electrodes 45 have to be provided which makes the size of the apparatus large.
As to unevenness of film deposited by CVD, it is also the problem in the case of plasma CVD. The energy of plasma seems dependent on the relationship between the substrate and a pair of electrodes for discharge. So a uniform deposition condition on a substrate to be coated is also demanded for plasma CVD.